1. Field of the Invention
The present invention relates to an improvement on a programmable electronic computer (hereinafter called programmable computer) which is capable of storing instructions, calculating equations, etc., for example through key operations on a keyboard.
2. Description of the Prior Art
In the conventional programmable computer, the instructions, calculating equations, etc., which are stored in a program storage, for example, from a keyboard, are coded into uniquely corresponding one-step instruction codes, which are read afterwards for the execution of such instructions. In recent years, however, the development of large-scale programmable computers of higher performance have resulted in the use of a more complicated instruction scheme in which one instruction cannot sometimes by represented by a one-step instruction code but is represented by plural steps, so that the instructions of one, two or more steps are mixedly stored in the memory. However, in such system of storing instructions of different lengths, it becomes difficult to simply determine the top address of an instruction represented by plural steps, as in the case of the simple memory address system involving one or two-step instructions alone, and a careless addressing of the program sequences leads to the read out or execution of a code erroneously recognized as an instruction instead of a non-instruction part, thus resulting in an erroneous operation or calculation.